function [mssim, ssim_map] = ssim_index(img1, img2, K, window, L)
% ========================================================================
% SSIM Index, Version 1.0
% Copyright(c) 2003 Zhou Wang
% All Rights Reserved.
% 
% The author is with Howard Hughes Medical Institute, and Laboratory
% for Computational Vision at Center for Neural Science and Courant
% Institute of Mathematical Sciences, New York University.
% 
% ----------------------------------------------------------------------
% Permission to use, copy, or modify this software and its documentation
% for educational and research purposes only and without fee is hereby
% granted, provided that this copyright notice and the original authors'
% names appear on all copies and supporting documentation. This program
% shall not be used, rewritten, or adapted as the basis of a commercial
% software or hardware product without first obtaining permission of the
% authors. The authors make no representations about the suitability of
% this software for any purpose. It is provided "as is" without express
% or implied warranty.
% ----------------------------------------------------------------------
% 
% This is an implementation of the algorithm for calculating the
% Structural SIMilarity (SSIM) index between two images. Please refer
% to the following paper:
% 
% Z. Wang, A. C. Bovik, H. R. Sheikh, and E. P. Simoncelli, "Image
% quality assessment: From error measurement to structural similarity"
% IEEE Transactios on Image Processing, vol. 13, no. 1, Jan. 2004.
% 
% Kindly report any suggestions or corrections to zhouwang@ieee.org
% 
% ----------------------------------------------------------------------
% 
% Input : (1) img1: the first image being compared
%        (2) img2: the second image being compared
%        (3) K: constants in the SSIM index formula (see the above
%            reference). defualt value: K = [0.01 0.03]
%        (4) window: local window for statistics (see the above
%            reference). default widnow is Gaussian given by
%            window = fspecial('gaussian', 11, 1.5);
%        (5) L: dynamic range of the images. default: L = 255
% 
% Output: (1) mssim: the mean SSIM index value between 2 images.
%            If one of the images being compared is regarded as
%            perfect quality, then mssim can be considered as the
%            quality measure of the other image.
%            If img1 = img2, then mssim = 1.
%        (2) ssim_map: the SSIM index map of the test image. The map
%            has a smaller size than the myinput images. The actual size:
%            size(img1) - size(window) + 1.
% 
% Default Usage:
%   Given 2 test images img1 and img2, whose dynamic range is 0-255
% 
%   [mssim ssim_map] = ssim_index(img1, img2);
% 
% Advanced Usage:
%   User defined parameters. For example
% 
%   K = [0.05 0.05];
%   window = ones(8);
%   L = 100;
%   [mssim ssim_map] = ssim_index(img1, img2, K, window, L);
% 
% See the results:
% 
%   mssim                        %Gives the mssim value
%   imshow(max(0, ssim_map).^4)  %Shows the SSIM index map
% 
% ========================================================================

%%
mssim = -Inf;
ssim_map = -Inf;
nIn = nargin;

if (nIn < 2 || nIn > 5), return; end
if any(size(img1) ~= size(img2)), return; end
[M, N] = size(img1);

% default settings
if nIn < 3, K = [0.01 0.03]; end
if nIn < 4,  window = fspecial('gaussian', 11, 1.5); end
if nIn < 5,  L = 255; end

if M < 11 || N < 11, return; end
if length(K) ~= 2 || any(K) < 0, return; end

[H, W] = size(window);
if ((H*W) < 4 || (H > M) || (W > N)), return; end

%%
C1 = (K(1)*L)^2;
C2 = (K(2)*L)^2;
window = window/sum(sum(window));
img1 = double(img1);
img2 = double(img2);

mu1   = filter2(window, img1, 'valid');
mu2   = filter2(window, img2, 'valid');
mu1_sq = mu1.*mu1;
mu2_sq = mu2.*mu2;
mu1_mu2 = mu1.*mu2;
sigma1_sq = filter2(window, img1.*img1, 'valid') - mu1_sq;
sigma2_sq = filter2(window, img2.*img2, 'valid') - mu2_sq;
sigma12 = filter2(window, img1.*img2, 'valid') - mu1_mu2;

if (C1 > 0 && C2 > 0)
    ssim_map = ((2*mu1_mu2 + C1).*(2*sigma12 + C2))./((mu1_sq + mu2_sq + C1).*(sigma1_sq + sigma2_sq + C2));
else
    numerator1 = 2*mu1_mu2 + C1;
    numerator2 = 2*sigma12 + C2;
    denominator1 = mu1_sq + mu2_sq + C1;
    denominator2 = sigma1_sq + sigma2_sq + C2;
    ssim_map = ones(size(mu1));
    index = (denominator1.*denominator2 > 0);
    ssim_map(index) = (numerator1(index).*numerator2(index))./(denominator1(index).*denominator2(index));
    index = (denominator1 ~= 0) & (denominator2 == 0);
    ssim_map(index) = numerator1(index)./denominator1(index);
end

mssim = mean2(ssim_map);

return